Casting Process

• Foundry = factory equipped for making molds, melting and handling

  molten metal, performing the casting process, and cleaning the finished

  casting.
• The workers who perform casting are called foundrymen.
• Casting

  It is a process which involves:

  1. pouring molten metal into a mold

  patterned after the part to be

  manufactured,

  2. allowing it to cool, and

  3. removing the metal from the

  mold.
• The Mold

  • Contains cavity whose geometry determines part shape
• Actual size and shape of cavity must be slightly oversized to allow

  for shrinkage of metal during solidification and cooling.
• Expendable Molds

  They are made of sand, plaster, ceramics, and

  similar refractory materials.

  Mixed with binders/ bonding agents

  Hence, they are called expendable; meant to be broken to get

  the product inside.
• Binders / Bonding agents – used to provide cohesiveness and

  structural stability.
• Permanent

  Molds

  They are made of metals which

  maintain their strength at high

  temperatures

  They can be used repeatedly due

  to their strength.
• Composite molds

  They are made of two or more different materials; such as sand,

  graphite, and metals.

  Improve mold strength, control cooling rates, and optimize process

  economics.
• Sand Casting

  It is the traditional method of casting, making it to be the most

  prevalent even up to this day.

  Most of the molding processes use Silica sand (SiO2) as the mold

  material because it resists high temperatures.

  It is also added with binders such as clays, zircon, olivine, etc.
• TWO GENERAL TYPES OF SAND:

  1. Naturally bonded – bank sand

  2. Synthetic – lake sand
• In order to determine the proper sand mold to be selected, certain

  factors are to be considered:

  1. Surface finish (fine grains form smooth mold surface)

  2. Good permeability (allows steam and gases to escape)

  3. Good collapsibility (allow shrinking while cooling)
• Open mold - simply a container in the shape of the desired part

  Closed mold - mold geometry is more complex and requires a gating

  system (passageway) leading into the cavity.
• Cold-box

  Organic/inorganic binders are

  blended for better

  dimensional accuracy.

  More expensive than green

  sand.
• Green-sand

  Sand + Clay + Water

  ‘green’ – sand is moist

  Least expensive of making molds
• No-bake molds

  Synthetic liquid resin mixed with sand

  Hardens at room temperature
• Terms:

  A) Flask

  B) Cope

  C) Drag

  D) Core

  E) Core print

  
• Terms:

  A) Mold Cavity

  B) Riser

  C) Gating System

  D) Pouring cup

  E) Sprue

  
• Terms:

  A) Runners

  B) Parting Line

  C) Draft

  D) Core Box

  
• Heating furnaces are used to

  heat the metal to molten

  temperature sufficient for

  casting.

  The heat required is the sum of:

  1. Heat to raise temperature to

  melting point

  2. Heat of fusion to convert from

  solid to liquid

  3. Heat to raise molten metal to

  desired temperature for pouring
• For this step to be successful, metal

  must flow into all regions of the mold, most

  importantly to the main cavity, before

  solidifying.

  Factors that determine success

  Pouring temperature – must be higher

  than solidification temperature

  Pouring rate

  Turbulence – inconsistent and irregular

  variations in the flow
• Solidification of Metals

  Transformation of molten metal back into solid state.

  Solidification differs depending on whether the metal is:

  • Pure metal
  • Alloy
• Due to chilling action of mold wall, a thin skin of solid metal is

  formed at the interface immediately after pouring.

  Skin thickness increases to form a shell around the molten metal

  as solidification progresses.

  Rate of freezing depends on heat transfer into mold, as well as

  thermal properties of the metal.
• Solidification

  A) ALLOYS

  
• Solidification:

  A) Pure Metals

  
• Formula:

  
• Solidification Shrinkage

  Occurs in nearly all metals because the solid phase has a higher

  density than the liquid phase.

  Exception: cast iron with high C content (gray iron, 2.5% – 4% Carbon)

  Graphitization during final stages of freezing causes expansion that counteracts

  volumetric decrease associated with phase change.
• Patternmakers account for solidification shrinkage and thermal

  contraction by making mold cavity oversized.

  Amount by which mold is made larger relative to final casting size is

  called pattern shrinkage allowance.

  Casting dimensions are expressed linearly, so allowances are applied

  accordingly.
• Riser is a waste material

  Separated from the casting and

  remelted to make more castings

  To minimize waste in the unit

  operation, it is desirable for the

  volume of metal in the riser to be

  a minimum

  A riser must have longer freezing

  time (small surface area per unit

  volume) than the casting.
• Segregation Defects

  It happens when the

  concentration of the

  solute is not constant

  throughout the casting.

  Example : Sn-Cu alloy

  Segregation can be

  reduced by

  homogenization

  (process of making things

  uniform or similar)

  The homogenization can

  be achieved through

  faster cooling rate.
• Porosity - caused by shrinkage, trapped gases, or

  both.

  Porosity is detrimental to ductility and fatigue life –

  must be kept to minimum levels.

  Shrinkage induced porosity occurs when molten

  metal is trapped inside already solidified metal
• Shrinkage induced porosity can take

  on odd shapes; i.e., they need not be

  spherical.

  Gas porosity is caused by gas

  bubbles. Initially, the gases are

  dissolved in the molten metal.
• Shell Mold Casting

  The box contains fine

  sand, mixed with

  thermosetting binder

  (phenol-formaldehyde)

  that coats the sand

  particles.

  The sand will coat the

  pattern.
• Investment Casting

  AKA LOST-WAX process

  “Investment” is derived

  from the fact that the

  pattern is invested with the

  refractory material

  Refractory - has an

  unusually high melting

  point and that maintains

  its structural properties

  at very high

  temperatures.

  Wax can still be recovered

  and reused